The present invention relates to objectives.
Many products necessitate compact and light objectives having a high imaging performance, which should, however, be cost-effective to produce.
Depending on requirements, several lenses and lens groups having different optical characteristics are necessitated in conventional objectives, for example to sufficiently correct geometric imaging errors or color errors. For lowering production costs, glass lenses can be produced, besides conventional grinding and polishing methods, for example in blank molding technology. Still, cost-intensive mechanical lens mounts and an assembly with different processing steps are necessitated.
Further, lenses can be produced cheaply in high quantities with plastic forming technology. The material shrinkage resulting during the forming process, which leads to homogeneity variations of the material, local modifications of the refraction index or shape deviations in the optical areas, can mostly be compensated with corrective free-form surfaces in the molding tool.
For correcting, for example, the color error of an objective with little optical effort, it is possible to use purely reflective optical systems. Depending on the design, these objectives are very sensitive to production and assembly tolerances, such that the same are unsuitable for large volumes.
To combine the advantages of reflective and refractive systems and at the same time to lower production costs, so-called monolithic objectives having reflective and refractive functional areas are known (EP 0 921 427 B1, DE 696 24 021 T2). They consist of a full body transparent for part of the electromagnetic spectrum having respective functional areas on its surface. Forming these monolithic systems is thereby mostly achieved directly in one production step, such as injection molding. Accordingly, with such systems, the number of optical elements can be limited to one, the assembly and adjustment effort can be reduced significantly and, hence, costs can be reduced to a certain extent. Further, no expensive mechanical mounts are necessitated since the monolithic objective includes all optical functional areas in a self-supporting manner.
To be able to produce monolithic systems in plastic forming technology, molds have to be provided having several angularly arranged high-precision aspheric areas or also free-form areas. These molds are produced with common multi-axes CNC ultra-precision machine tools as one component or from several ultra-precisely assembled parts.
Despite the ultra-precision processing technologies common nowadays, mold production costs increase with every optical functional area and its mold complexity. The systems in EP 0 921 427 B1 and DE 696 24 021 T2 have at least seven highly complex free-form areas mathematically described by higher-order polynomials. This increases production costs and additionally reduces the light intensity of the objective due to absorption losses. Additionally, the mathematical description the optical functional area is complex, which makes production of the same more difficult and more cost-intensive.
The embodiments shown in EP 0 921 427 B1 and DE 696 24 021 T2 all comprise intermediate imaging, which lengthens the optical path and hence makes the structure more bulky. Additionally, in the embodiments of EP 0 921 427 B1 and DE 696 24 021 T2, the system apertures simultaneously serving as entrance pupils are arranged in the optical path in front of the first optical functional element. However, with a system aperture arranged outside the objective it is very difficult to correct imaging errors due to the lack of symmetry within the system.
Further, the embodiments of the above-stated patent literature show an arrangement of the image sensor with an air gap to the last, mostly bent optical functional area, so that additional and cost-intensive mechanical components are necessitated for alignment and permanent fixing of the image sensor. This increases the difficulty of mounting the sensor exactly and with few tolerances.
Frequently, PMMA or Zeonex® is used forming material for monolithic plastic systems.
The anti-reflective and rear-side mirroring layers necessitated for the monolithic objective can be efficiently deposited in several coating processes with common large-volume plants.
Although the above solutions already individually show several advantages, a compact objective without intermediate images would be desirable, which is not only shock resistant, has a high imaging quality and can be produced in large volumes at low costs, but which also comprises a large image field of more than ±25° on the diagonal, an initial opening of less than F4.5, a small number of optical functional areas and a simple, exact and cost-effective assembly option for the image sensor.